1. Field of the Invention
The present invention relates to a display device, a method of laying out wiring in a display device, and an electronic device, and particularly to a flat-panel type display device in which pixels including an electrooptic element are arranged two-dimensionally in the form of a matrix, a method of laying out wiring in the display device, and an electronic device having the display device.
2. Description of the Related Art
Recently, flat-panel type display devices in which pixels (pixel circuits) including a light emitting element are arranged in the form of a matrix have been spreading rapidly in a field of display devices displaying images. The development and commercialization of a flat-panel type display device using a so-called current-driven type electrooptic element changing in light emission luminance depending on the value of current flowing through the device as the light emitting element of a pixel, for example an organic EL display device using an organic EL (Electro Luminescence) element utilizing a phenomenon of light being emitted when an electric field is applied to an organic thin film as the light emitting element of a pixel have been under way.
The organic EL display device has the following features. The organic EL element can be driven by an application voltage of 10 V or lower, and thus consumes low power. Because the organic EL element is a self-luminous element, as compared with a liquid crystal display device that displays an image by controlling the intensity of light from a light source (backlight) in a liquid crystal in each pixel, the organic EL display device provides high image visibility, and is easily reduced in weight and thickness because an illuminating member such as a backlight is not required. Further, because the organic EL element has a very high response speed of a few μsec or so, no afterimage occurs at a time of displaying a moving image.
As with the liquid crystal display device, the organic EL display device can adopt a simple (passive) matrix system and an active matrix system as a driving system of the organic EL display device. However, while having a simple structure, a simple matrix type display device presents, for example, a problem of difficulty in realizing a large and high-definition display device because the emission period of an electrooptic element is reduced by an increase in the number of scanning lines (that is, the number of pixels).
Therefore, an active matrix type display device that controls current flowing through an electrooptic element by an active element, for example an insulated gate field effect transistor (typically a TFT (Thin Film Transistor)) provided within a same pixel circuit as the electrooptic element has recently been actively developed. The active matrix type display device makes it easy to realize a large and high-definition display device because the electrooptic element continues emitting light over the period of one frame.
It is generally known that the I-V characteristic (current-voltage characteristic) of the organic EL element is degraded with the passage of time (so-called secular degradation). In a pixel circuit using an N-channel type TFT as a transistor that current-drives an organic EL element (which transistor will hereinafter be described as a “driving transistor”), when the I-V characteristic of the organic EL element is degraded with the passage of time, the gate-to-source voltage Vgs of the driving transistor changes, because the organic EL element is connected to the source electrode side of the driving transistor. As a result, the light emission luminance of the organic EL element also changes.
This will be described more specifically. The source potential of the driving transistor is determined by an operating point of the driving transistor and the organic EL element. When the I-V characteristic of the organic EL element is degraded, the operating point of the driving transistor and the organic EL element varies. Thus, even when a same voltage is applied to the gate of the driving transistor, the source potential of the driving transistor changes. Thereby, the gate-to-source voltage Vgs of the driving transistor changes, and therefore the value of current flowing through the driving transistor changes. As a result, the value of current flowing through the organic EL element also changes, so that the light emission luminance of the organic EL element changes.
Further, in a pixel circuit using a polysilicon TFT in particular, in addition to a secular degradation in the I-V characteristic of an organic EL element, there may occur secular changes in threshold voltage Vth of a driving transistor and in mobility μ of a semiconductor thin film forming the channel of the driving transistor (which mobility will hereinafter be described as “mobility of the driving transistor”), and there may be a difference in the transistor characteristics of the threshold voltage Vth and the mobility μ in each pixel due to variations in a manufacturing process (there are variations between the transistor characteristics of individual pixels).
When the threshold voltage Vth and the mobility μ of the driving transistor differ in each pixel, the value of current flowing through the driving transistor varies in each pixel. Thus, even when a same voltage is applied to the gate electrodes of driving transistors in respective pixels, the light emission luminance of the organic EL element varies between the pixels. As a result, screen uniformity is impaired.
Accordingly, in order to hold the light emission luminance of the organic EL element constant without being affected by a secular degradation in the I-V characteristic of the organic EL element or a secular change in the threshold voltage Vth or the mobility μ of the driving transistor even when the secular degradation occurs in the I-V characteristic of the organic EL element or the secular change occurs in the threshold voltage Vth or the mobility μ of the driving transistor, a constitution is adopted which constitution provides each of pixel circuits with a function of compensating for variations in the characteristic of the organic EL element and correcting functions of correcting for variations in the threshold voltage Vth of the driving transistor (which correction will hereinafter be described as “threshold value correction”) and correcting for variations in the mobility μ of the driving transistor (which correction will hereinafter be described as “mobility correction”) (see Japanese Patent Laid-Open No. 2006-133542 (hereinafter referred to as Patent Document 1), for example).
By thus providing each of the pixel circuits with the function of compensating for variations in the characteristic of the organic EL element and the correcting functions of correcting for variations in the threshold voltage Vth and the mobility μ of the driving transistor, the light emission luminance of the organic EL element can be held constant without being affected by a secular degradation in the I-V characteristic of the organic EL element or a secular change in the threshold voltage Vth or the mobility μ of the driving transistor even when the secular degradation occurs in the I-V characteristic of the organic EL element or the secular change occurs in the threshold voltage Vth or the mobility μ of the driving transistor. Therefore, the display quality of the organic EL display device can be improved.
The related techniques described in Patent Document 1 provide each of the pixel circuits with the function of compensating for variations in the characteristic of the organic EL element and the correcting functions of correcting for variations in the threshold voltage Vth and the mobility μ of the driving transistor. Thereby, the light emission luminance of the organic EL element can be held constant without being affected by a secular degradation in the I-V characteristic of the organic EL element or a secular change in the threshold voltage Vth or the mobility μ of the driving transistor even when the secular degradation occurs in the I-V characteristic of the organic EL element or the secular change occurs in the threshold voltage Vth or the mobility μ of the driving transistor.